I think it could technically cause an issue because the tokens contain subs which close on the environment around them; if someone used two of the parsers in different contexts, the latter one's state might end up being used for the former, too!

So far I've tried to 'solve' this just by encapsulating these objects well enough such that they're never exposed to the user, so I deal with these issues in the interface (though the module becomes non-reentrant).